1. Field of the Invention
This invention relates generally to the area of conservation of power in a receiver, and more particularly to a method and apparatus for conserving power during a portion of the reception of a signal having a predetermined signal occurring at predetermined intervals.
2. Description of the Prior Art
A digital signalling protocol proposed by British Telecom in England is commonly termed POCSAG (Post Office Code Standardization Advisory Group) and provides a signal having a predetermined signal occurring at predetermined intervals. FIG. 1 shows a typical POCSAG protocol signal 10 and a power conservation strobe signal waveform 20 used in prior art receivers. The POCSAG signalling protocol is composed of batches wherein each batch includes a sync code 12 and eight frames of information 21-28. Each frame has two information words. An information word is a 32 bit binary word having 21 information bits and 11 parity bits. The information word is structured as a 31,21 extended BCH word having one extra parity bit, henceforth referred to as a 32,21 BCH word.
Area 30 of FIG. 1 represents a time when no signal is transmitted. The POCSAG signal begins with a preamble 32 which consists of an alternating pattern of logic "1" and "0" levels. The first batch following the preamble begins with a first sync code 12a. During the time spanning intervals 30, 32 and 12a, the receiver performs a sync acquisition process as indicated by area 52a of line 20 during which it first establishes bit sync with the signal and then establishes word or frame sync using processes well known in the art. After acquiring sync, the receiver begins a batch decoding process wherein the receiver decodes information within a preassigned frame. The information within the frame may include an address matching a preassigned address, in response to which the receiver would alert.
Assume the receiver operating per FIG. 1 has been preassigned to frame 4. Having acquired sync at the end of interval 52a, the receiver conserves power during interval 53a. The receiver operates in a fully operational, high power mode during interval 54a in order to decode information within frame 4. During interval 55a, the receiver operates in a low power, non-decoding mode until interval 56b wherein the receiver again operates in a high power mode in order to receive the second sync code 12b.
The power conservation cycle repeats for intervals 53b, 54b, 55b, and 56c. However, at interval 56c, the POCSAG signal 10 is terminated as indicated by the area 40. Termination of the signal occurs either when the POCSAG transmission is complete or when the signal is interrupted by noise. For the purpose of reference, the signal to which the receiver is synchronized is shown during interval 40 even though no signal is present. The receiver is not able to determine in interval 56c whether the sync code was not found because the transmission was completed, or because the signal interrupted by noise. Consequently, the receiver assumes the transmission is not complete and a noise interruption occurred. The receiver continues to search for information in interval 54c and sync code in interval 56d. Since in this example, the signal transmission has been completed, no information is received during interval 54c and no sync code is received during interval 56d. Upon determining that no sync code was received in two consecutive sync code intervals 56c and 56d, the receiver determines that the transmission has been completed and resumes the sync acquisition process as shown by area 52b.
Note that during interval 56b, the receiver operates in a high power mode for the entire duration of the sync code in order to determine the presence of the sync code 12b. The contents of the sync code is predetermined and generally, prior art receivers test for the termination of the transmission during the sync code.
With the growing success of digital paging systems, the length of data transmission signals has increased. FIG. 1 shows a two batch POCSAG signal. Oftentimes, POCSAG signals can be hundreds of batches long. In view of this and the ever decreasing size of paging receivers, batteries and consequently battery capacity, it is desirable to conserve power to the greatest extent possible during such transmissions. Additional conservation of power may be realized by conserving power during the transmission of predetermined signals such as the sync code signal used in the POCSAG protocol.